This paper reports an insect mountable biofuel cell (imBFC) which generates electric power from trehalose found in insect hemolymph and refuel trehalose from insect hemolymph by diffusion automatically. First, we designed and fabricated the imBFC consisted of a connector, two chambers, a dialysis membrane and electrodes. Then, we evaluated the power density and life-time of the imBFC. The maximum power of 50.2 μW (42.9 μW/cm2) was obtained. The power output of the imBFC was maintained at more than 10 μW for 3 h. Finally, we connected a light-emitting diode (LED) device to the imBFC and succeeded to flashing the LED. The results indicate that the imBFC is a promising micro battery to power environmental monitoring micro-tools.